The present invention relates to a thin film forming method and, for example, to (Ba, Sr)Ti3 (barium strontium titanate: to be referred to as BST hereinafter) used in, e.g., a semiconductor memory device.
As the semiconductor integrating technique develops rapidly, downsizing and increases in speed and integration degree have been achieved in various types of components constituting a semiconductor integrated device. For example, in the field of semiconductor memory devices, in addition to the issues described above, an increase in capacity has been required, and development in technique is very fast. This will be described using an example. In a DRAM (Dynamic Random Access Memory) as one of typical semiconductor memory devices, downsizing and an increase in capacitance per unit area are sought for in a capacitor as one of the main constituent elements. As a dielectric material generally used to form the capacitor of a DRAM, silicon dioxide is generally used from the viewpoint of the semiconductor process. Since silicon dioxide has a dielectric constant of 10 or less and a capacitance per unit area of 4 fF/xcexcm2, it cannot cope with a requirement for an increase in capacitance per unit area. For this reason, recently, BST and the like with a dielectric constant higher than that of silicon dioxide have received a great deal of attention.
The arrangement of the DRAM described above will be briefly described.
FIG. 13 shows some DRAM memory cells formed on an Si substrate 1301. Each DRAM memory cell is constituted by a transistor 1302 and capacitor 1310. The transistor 1302 is connected to the capacitor 1310 through a plug 1303 connected to the drain terminal. The plug 1303 is formed in a contact hole formed in an interlevel insulating film 1304 made of an insulator such as silicon dioxide.
The plug 1303 is made of polysilicon which is doped with an impurity so as to be conductive, and is connected to a storage node 1305 serving as one electrode of the capacitor 1310. The storage node 1305 is formed on the flat interlevel insulating film 1304 and is made of, e.g., a platinum, ruthenium, or ruthenium oxide film. The storage node 1305 is connected to the plug 1303 through a barrier film 1303a made of, e.g., TiN.
A capacitor film 1306 is formed on the interlevel insulating film 1304 including the storage node 1305. A cell plate 1307 is formed on the capacitor film 1306, and forms the capacitor 1310 together with the storage node 1305 and capacitor film 1306. An upper interlevel insulating film 1308 is formed on the capacitor 1310. Although not shown, a word line to be connected to the gate of the transistor 1302, a bit line to be connected to the source terminal, and the like are arranged on the upper interlevel insulating film 1308. As described above, the capacitor film is formed to cover one electrode formed on the transistor through the interlevel insulating film 1304 and serving as a capacitor.
As the requirements needed for this capacitor film serving as a capacitor, first, the capacitor film must have a high dielectric constant. Examples of the material with such a high dielectric constant are compounds containing Ba, Sr, Ti, Pb, Zn, Bi, Ta, or the like. Second, the capacitor film must have a small leakage current.
Even if a material with a high dielectric constant is used, a DRAM of 1 Gbit or more cannot be implemented by a planar capacitor, and it is indispensable to employ a stereoscopic structure, as described above. To form a stereoscopic capacitor, a thin film forming method is required which can form a film with a uniform thickness, composition, and characteristics on the flat portion and side wall of the storage node of the stereoscopic structure and which has a good coverage on a complicated stepped shape.
As a thin film forming technique with the good step coverage, chemical vapor deposition (CVD) is available. With CVD, the source of a material for forming a thin film to be formed must be transported onto a substrate in the form of a gas. The most preferable CVD source is the one that takes the form of a gas in room temperature. When a gas is used as the source, it can be controlled well because its supply amount is determined by only the gas flow rate. However, of the elements described above that form a high dielectric or ferroelectric, no one forms a compound which is gaseous at room temperature. Accordingly, when a material with a high dielectric constant is to be formed by CVD, the source is supplied by means of bubbling. More strictly, if the source is a solid source, it is supplied by sublimation.
When the source is to be supplied by bubbling, if the source gas is liquid, supply amount control can be performed more stably. The requirement for the source gas is a high vapor pressure, that is, if possible, it must have a sufficiently high vapor pressure at room temperature or less. However, almost none of the elements described above forms a compound with a sufficiently high vapor pressure, and most of compounds that have some vapor pressure are organometallic compounds.
From the above viewpoint, barium titanate (BaTiO3: BT), strontium titanate (SrTiO3: ST), and BST as a solid solution of BT and ST have received attention as the material of the capacitor film of a DRAM that can be formed with CVD described above.
More specifically, BT and ST are high dielectric constant materials with a high dielectric constant of about 100, and BST is a high dielectric constant material with a high dielectric constant of about 200 to 300, thus satisfying one performance described above which is required for the capacitor film. Barium, strontium, and titanium can form organic compounds, and a BT, ST, or BST thin film can be formed by MO (Metal organic) CVD. This thin film forming method is thermal CVD which forms a metal or compound film at a comparatively low temperature (400xc2x0 C. to 500xc2x0 C.) by using an organometallic compound (MO). In this manner, a BT, ST, or BST thin film can be formed with CVD that provides a good step coverage, and satisfies the second performance described above which is required for the capacitor film.
When forming a BST film by MOCVD, generally, a substrate as a thin film forming target is heated and, e.g., evaporated Ba(thd)2, Sr(thd)2, and Ti(Oxe2x80x94iPr)2(thd)2 are supplied onto the substrate, thus forming a BST film on the substrate. Ba(thd)2 is a barium source, Sr(thd)2 is a strontium source, and Ti(Oxe2x80x94iPr)2(thd)2 is a titanium source.
If the substrate is heated to an excessively high temperature during formation of the BST film described above, the step coverage suffers. When the substrate has a step with an aspect ratio of 3 or more, it is heated to a temperature of 450xc2x0 C. or less in order to set the step coverage to 90% or more. When the temperature is decreased, the film formation rate becomes low. Hence, when forming a BST film, the substrate is heated to about 450xc2x0 C.
When a BST film is to be formed by MOCVD, unless the temperature of the substrate as the thin film forming target becomes 500xc2x0 C. or more, the formed BST film cannot be set in a crystalline state wherein the formed BST film has a high dielectric constant enough use it as a capacitor film. More specifically, when a thin film is formed at a substrate temperature of about 450xc2x0 C. described above, the obtained BST film is in the amorphous state and is thus not appropriate as a capacitor film. Therefore, conventionally, a BST film in a crystalline state, which can be used as a capacitor film, is formed by three-step thin film formation to have good step coverage (reference: Japanese Patent Laid-Open No. 8-176826).
First, a thin strontium titanate film (ST film) is formed by using a predetermined MOCVD apparatus at a substrate temperature where desired step coverage can be obtained. Since crystal growth of the ST film occurs on a heterogeneous substrate more easily than a BST film, a comparatively thin initial film in a perovskite state with good crystallizability can be formed.
Second, by using another heating unit, the substrate where the thin ST film is formed is heated by an infrared heating lamp to promote crystallization of the thin ST film, so that a film to be formed on the ST film can be crystallized easily.
Third, by using the MOCVD apparatus again, a BST film in which the amount of Ba is larger than that of Sr is formed on the ST film. The layer of this BST film containing the large amount of Ba supplies Ba to the underlying ST film in the subsequent annealing.
Fourth, a BST film containing Sr and Ba at a rate of 1:1 is formed on the BST film in which the amount of Ba is larger than that of Sr. In this thin film formation, because of the presence of the underlying ST film as a crystal film, the BST film is formed in the crystalline state.
Fifth, by using another heating unit, the substrate where the three films are formed is heated by the infrared heating lamp to promote crystallization of the formed BST film. Simultaneously, Ba is supplied to the lowermost thin ST film, so that the entire structure forms a BST film in the crystalline state which can be entirely used as a capacitor film.
With the above procedures, a BST film can be formed with good step coverage as well as good crystallization.
Conventionally, many steps are required, as described above. Consequently, the system increases in size to increase the cost of the apparatus to be used for thin film formation, leading to a high product unit cost.
The present invention is made in order to solve the problems described above, and has as its principal object to form a crystal film of a metal oxide, e.g., BST, on the surfaces of different types of materials that form steps, with good step coverage while suppressing the cost.
In order to achieve the above object, according to an aspect of the present invention, as the first step, an atmosphere is set to a first vacuum degree, and a gas of an organometallic compound and oxygen gas are introduced onto a surface of a base heated to a first temperature, to form a plurality of crystal nuclei, which are made of an oxide of a metal constituting the organometallic compound, on the surface of the base. Subsequently, as the second step, the atmosphere is set to a second vacuum degree lower in vacuum than in the first step, and the gas of the organometallic compound and the oxygen gas are introduced onto the surface of the base heated to a second temperature, to form a crystal thin film made of an oxide of the metal on the surface of the base. In the first step, the first vacuum degree is set to a vacuum degree at which the oxide of the metal is formed by crystal growth on surfaces of different materials, and the plurality of crystal nuclei are formed at a high density so that a crystal grain which is formed by a crystal nucleus upon growing comes into contact with a crystal grain growing from an adjacent crystal nucleus. In the second step, the second temperature is set to less than a temperature at which the oxide of the metal is formed by crystal growth on the surfaces of the different materials. In the above two steps, as the organometallic compound, an organic compound of strontium, an organic compound of barium, and an organic compound of titanium are used. Alternatively, as the organometallic compound, the organic compound of titanium and one of the organic compound of strontium and the organic compound of barium are used. In the first step, a gas of the organic compound of titanium, the oxygen gas and one of a gas of the organic compound of strontium and a gas of the organic compound of barium are introduced onto the surface of the base. In the second step, the gas of the organic compound of strontium, the gas of the organic compound of barium, the gas of the organic compound of titanium, and the oxygen gas are introduced onto the surface of the base.
According to the present invention, after the crystal nuclei are formed, a crystal film of a metal oxide such as BST is formed at a temperature providing good step coverage and falling within a temperature range where a metal oxide is not formed by crystal growth on surfaces of different materials.
According to another aspect of the present invention, as the pre-process, a surface of a base as a thin film forming target is exposed to a plasma. After that, as the first step, an atmosphere is set to a first vacuum degree, and a gas of an organometallic compound and oxygen gas are introduced onto the surface of the base heated to a first temperature, to form a plurality of crystal nuclei, which are made of an oxide of a metal constituting the organometallic compound, on the surface of the base. Subsequently, as the second step, the atmosphere is set to a second vacuum degree, and the gas of the organometallic compound and the oxygen gas are introduced onto the surface of the base heated to a second temperature, to form a film made of an oxide of the metal on the surface of the base. In the first step, the first vacuum degree is set to such a vacuum degree at which the oxide of the metal is formed by crystal growth on the surface of the base, which has been subjected to the pre-process, at the first temperature, and the plurality of crystal nuclei are formed at a high density so that a crystal grain which is formed by growing a crystal nucleus comes into contact with a crystal grain growing from an adjacent crystal nucleus. In the second step, the second temperature is set to less than a temperature at which the oxide of the metal is formed by crystal growth on surfaces of different materials. In the above process, as the organometallic compound, an organic compound of strontium, an organic compound of barium, and an organic compound of titanium are used. As the organometallic compound, the organic compound of titanium and one of the organic compound of strontium and the organic compound of barium are used. In the first step, a gas of the organic compound of titanium, the oxygen gas and one of a gas of the organic compound of strontium and a gas of the organic compound of barium are introduced onto the surface of the base. In the second step, the gas of the organic compound of strontium, the gas of the organic compound of barium, the gas of the organic compound of titanium, and the oxygen gas are introduced onto the surface of the base.
According to the present invention, after the crystal nuclei are formed, a crystal film of a metal oxide such as BST is formed at a temperature providing good step coverage and falling within a temperature range where a metal oxide is not formed by crystal growth on surfaces of different materials.
According to still another aspect of the present invention, as the first step, an atmosphere is set to a first vacuum degree, and a gas of an organometallic compound and oxygen gas are introduced onto a surface of the base heated to a first temperature, to form a plurality of crystal nuclei, which are made of an oxide of a metal constituting the organometallic compound, on the surface of the base. As the second step, the atmosphere is set to a second vacuum degree, and the gas of the organometallic compound and the oxygen gas are introduced onto the surface of the base heated to a second temperature, to form a film made of the oxide of the metal on the surface of the base. In the first step, the first temperature is set to a temperature at which the oxide of the metal is formed by crystal growth on surfaces of different materials, and the plurality of crystal nuclei are formed at a high density so that a crystal grain which is formed by growing a crystal nucleus upon comes into contact with a crystal grain growing from an adjacent crystal nucleus. In the second step, the second temperature is set to less than a temperature at which the oxide of the metal is formed by crystal growth on the surfaces of the different materials. In the above process, as the organometallic compound, an organic compound of strontium, an organic compound of barium, and an organic compound of titanium are used. As the organometallic compound, the organic compound of titanium and one of the organic compound of strontium and the organic compound of barium are used. In the first step, a gas of the organic compound of titanium, the oxygen gas and one of a gas of the organic compound of strontium and a gas of the organic compound of barium are introduced onto the surface of the base. In the second step, the gas of the organic compound of strontium, the gas of the organic compound of barium, the gas of the organic compound of titanium, and the oxygen gas are introduced onto the surface of the base.
According to the present invention, after the crystal nuclei are formed, a crystal film of a metal oxide such as BST is formed at a temperature providing good step coverage and falling within a temperature range where a metal oxide is not formed by crystal growth on surfaces of different materials.
Another aspect of the present invention comprises the first step of setting an atmosphere for a base as a thin film forming target to a predetermined vacuum degree, and introducing a gas of an organometallic compound, an oxidizing gas, and an inert gas onto a surface of the base heated to a first temperature, to form a plurality of crystal nuclei, which are made of an oxide of a metal constituting the organometallic compound, on the surface of the base, and the second step of setting the atmosphere for the base to a predetermined vacuum degree, and introducing the gas of the organometallic compound and the oxidizing gas onto the surface of the base heated to a second temperature, to form a film made of the oxide of the metal on the surface of the base. In the first step, a partial pressure of the organometallic compound on the base is set to a vacuum degree at which the oxide of the metal is formed by crystal growth on surfaces of different materials at the first temperature, and the plurality of crystal nuclei are formed at a high density so that a crystal grain which is formed by growing a crystal nucleus comes into contact with a crystal grain growing from an adjacent crystal nucleus. In the second step, the second temperature is set to less than a temperature at which the oxide of the metal is formed by crystal growth on the surfaces of the different materials.
According to the present invention, after the crystal nuclei are formed by decreasing the partial pressure of the organometallic compound on the base, a crystal film of a metal oxide such as BST is formed at a temperature providing good step coverage and falling within a temperature range where a metal oxide is not formed by crystal growth on surfaces of different materials.